Disambiguate PHP shortform ternary

This should fix #56 

The current PHP parsing did not support short from ternary (`?:`) with an immediate token such as `true` or `false` following it. I copied the ruleset from https://github.com/facebook/hhvm/blob/master/hphp/parser/hphp.ll#L515 (except for the _maybe_ states) and tried to implement a better lookahead. 

I am not sure I did this in the correct/most efficient way and welcome comments/suggestions.